I think this was her debut Hollywood feature film so she had plenty of time to develop as an actress. It was the same thing with George Lazenby with OHMSS being his first ever film and after that, even if he didn't become the big star he was supposed to be, he did quite a lot of films including some HK actioners.
At the moment, I'm reading Mute Witness by Robert L. Pike, the 1963 novel that Bullitt is based on. The main character is called Lt. Clancy, the story's set in New York, the baddie is called Rossi and instead of Delgetti, Bullitt's right hand man is called Kaproski and he's a big guy. Clancy talks a lot more than Bullitt and has more of a bad temper and a bit of backstory - before Chalmers gave him the Rossi assignment, he had it in for Clancy anyway because he had shot dead his witness. That must have inspired the airport finale of the movie.